The application of gene therapy to the treatment of AIDS and HIV infection has been discussed widely (14). The types of therapeutic gene proposed usually fall into one of two broad categories. In the first the gene encodes protein products that inhibit the virus in a number of possible ways. One example of such a protein is the RevM10 derivative of the HIV Rev protein (16). The RevM10 protein acts as a transdominant negative mutant and so competitively inhibits Rev function in the virus. Like many of the protein-based strategies, the RevM10 protein is a derivative of a native HIV protein. While this provides the basis for the anti-HIV effect, it also has serious disadvantages. In particular, this type of strategy demands that in the absence of the virus there is little or no expression of the gene. Otherwise, healthy cells harbouring the gene become a target for the host cytotoxic T lymphocyte (CTL) system, which recognises the foreign protein (17, 25). The second broad category of therapeutic gene circumvents these CTL problems. The therapeutic gene encodes inhibitory RNA molecules; RNA is not a target for CTL recognition. The RNA molecules may be anti-sense RNA (15, 31), ribozymes (5) or competitive decoys (1).
Ribozymes are enzymatic RNA molecules which catalyse sequence-specific RNA processing. The design and structure of ribozymes has been described extensively in the literature in recent years (3, 7, 31). Amongst the most powerful systems are those that deliver multitarget ribozymes that cleave RNA of the target virus at multiple sites (5, 21).
In recent years a number of laboratories have developed retroviral vector systems based on HIV (2, 4, 18, 19, 22-24, 27, 32, 35, 39, 43). In the context of anti-HIV gene therapy these vectors have a number of advantages over the more conventional murine based vectors such as murine leukaemia virus (MLV) vectors. Firstly, HIV vectors would target precisely those cells that are susceptible to HIV infection (22, 23). Secondly, the HIV-based vector would transduce cells such as macrophages that are normally refractory to transduction by murine vectors (19, 20). Thirdly, the anti-HIV vector genome would be propagated through the CD4+ cell population by any virus (HIV) that escaped the therapeutic strategy (7). This is because the vector genome has the packaging signal that will be recognised by the viral particle packaging system. These various attributes make HIV-vectors a powerful tool in the field of anti-HIV gene therapy.
A combination of the multitarget ribozyme and an HIV-based vector would be attractive as a therapeutic strategy. However, until now this has not been possible. Vector particle production takes place in producer cells which express the packaging components of the particles and package the vector genome. The ribozymes that are designed to destroy the viral RNA would therefore also interrupt the expression of the components of the HIV-based vector system during vector production. The present invention aims to overcome this problem.